There is much commonality between currently available large mining trucks made by different manufacturers and the following observations are generally applicable regardless of the truck manufacturer:
The empty vehicle weight is a high proportion of the maximum gross vehicle weight. Typically the ratio of payload to empty vehicle weight is only about 1.4:1. This means that much of the cost of operating such trucks is related to moving the empty vehicle weight rather than the payload. PA1 The total width of the four rear tires is large compared to the total width of the truck. Typically 65% of the total width of a truck is taken up by the four rear tires. With present designs of truck this leads to a very narrow main frame for the truck and very high bending loads on the rear axle and rear wheel support systems. The narrow main frame causes shortage of space for maintenance of some components, high stress changes during cornering maneuvers, the need for vertically stiff rear suspension springs and design restrictions on the body. The net effect is high weight and cost for the main frame, the rear axle, the rear wheel support assemblies and the body. PA1 The travel of the rear suspension system of a fully loaded truck is very limited compared to the scale of the truck. Typically the maximum travel in the compression direction of the rear axle relative to the main frame is only of the order of 50 mm when loaded. This limited travel is a result of the need to achieve adequate roll stiffness from the two narrowly spaced rear spring units. PA1 The main frame of these trucks are complex welded steel structures that are heavy (e.g. 16.5 tons for the main frame of a truck with a payload rating of 172 tons), expensive to design, develop and manufacture, and prone to fatigue cracking. PA1 The main load carrying member (the body) of the trucks is a very strong and generally stiff member. This strength and stiffness is a consequence of the need for the body to withstand the shock loads applied during loading of large rocks by large excavators. PA1 The body is generally supported by the main frame of the truck at numerous points. For example at the rear pivot points, at two, four, six or eight points along the underside of the body and in some trucks also at forward extensions of the body which contact the main frame at points which are close to being above the line joining the centers of the front wheels. This system of supporting the stiff body causes high variations of stress levels in the main frame of the truck and the body as the truck traverses over uneven ground and during cornering. This feature causes fatigue problems, high design and fabrication costs and the need for considerable expenditure to limit the unevenness of the ground on which the trucks travel. PA1 The body is tipped (hoisted) by hydraulic cylinders which react against the main frame of the truck at points near to midway between the front and rear wheels. This causes very large bending loads to be applied to the main frame of the truck and requires that the main frame by very massive at the mid sections. It can also cause large stress changes in the body. PA1 The dual rear tires are rotationally locked together. During short radius turning maneuvers (frequent occurrences in typical mining operations), this causes severe scrubbing type wear of the tires due to the differential travel distance effect. Relative scrubbing between the two tires of a dual set is considered to contribute significantly to total wear of rear tires on large mining trucks. PA1 The combination of four wide tires on a solid beam type rear axle causes large variations in individual tire loads as the truck traverses uneven ground conditions. This arrangement also means that it is necessary to take considerable care in matching tire outside diameters and inflation pressures to minimize the unevenness in tire loads on level ground conditions. PA1 In general with currently available truck designs, the transfer of forces between the body and the ground is through a very indirect path which involves high bending loads in the body, the main frame of the truck, the rear axle housing and the rear wheel support systems. Furthermore these bending loads fluctuate greatly as the truck travels over uneven ground, during cornering and when the dump body is raised during unloading. PA1 a substantially rigid load support body, means for pivotally mounting said body with respect to said main frame in proximity of the pivotal mounting of the suspension means to said frame member, and means for maintaining transverse spacing between said rear wheels.
A typical very large mining truck is shown in FIG. 1 of the drawings of U.S. Pat. No. 5,385,391, and it will be noted that the frame structure is quite substantial and this results from the frame being required to bear the load supported by the body of the truck by contact between the body and the upper surfaces of the frame, and by virtue of the body hoisting rams being connected to the frames as shown. The substantial size of the rear axle is also apparent.
While many truck frame design improvements have been suggested over the years, no one design has successfully addressed more than a few of the difficulties which have been outlined above. For example, U.S. Pat. No. 3,704,040 to Davis et al discloses a frame arrangement in which the front and rear wheel pairs are centrally supported. The independent frame members arrangement which is described as addressing many of the problems created by uneven terrain is extremely complex and consequently expensive and heavy. This patent and the related U.S. Pat. No. 3,773,348 also disclose a rear suspension arrangement suitable for use with centrally supported rear wheel pairs. Centrally supported rear wheel pairs provide the potential to overcome some of the problems inherent in currently available large mining trucks, but to date no successful method of exploiting this potential has been established.
In addition to the above, most truck body designs have remained essentially unchanged for many years, being characterized by extremely heavy structures reinforced by means of relatively closely spaced transverse beams assisted by limited longitudinal beams, thereby resulting in a body structure of extremely high weight.
Similarly, most conventional truck bodies have a sloping floor and vertical longitudinal sides arranged at a constant width spacing. In one departure from this approach, a body having a flat floor and vertical sides which are wider apart at the rear of the body than at the front of the body was designed. Although in this arrangement, the wear on the sides of the body is reduced, the flat body floor is not compatible with most truck main frame designs and it increases the height of the center of gravity of the truck unless the overall width and/or length of the truck is increased.
At least some of the problems outlined above are overcome in the very large vehicle described in U.S. Pat. No. 5,476,285.
FIGS. 1 to 7 of the drawings show the truck frame construction of U.S. Pat. No. 5,476,285. As shown in FIGS. 1-5, the truck frame comprises a pair of relatively lightweight longitudinal members 10 and 11 suspending rear wheel mounting hubs 12 and 13 towards their rear ends. The longitudinal members 10 and 11 are connected at their forward ends by a substantial cross member 14, such as a mounting collar which generally includes mounting points for the front suspension (not shown) for the front wheels W.sub.f, at the front by a bumper 60 and at the rear, forwardly of the rear wheels, by a cross member 60a, which forms part of the support means for the rear wheel mounting hubs 12 and 13, as will be described further below. The front suspension may take the form described in greater detail in U.S. Pat. No. 5,385,391 or in any other suitable form.
As shown in FIGS. 3, 4 and 5, the wheel mounting hubs 12 and 13 support drive means for the rear wheels W.sub.r, such as electric traction motors 16, 17, 18 and 19, which are in turn connected to the rear wheels W.sub.r, with each wheel being mounted one on either side of the hubs 12 and 13. The location of the rear wheel mounting hubs 12 and 13 between each pair of rear wheels W.sub.r allows independent rotation of each wheel thereby avoiding tire wear caused by short radius turning.
The rear suspension system of FIGS. 1-6 includes rear wheel mounting hubs 12 and 13 mounted to the longitudinal frame members 10 and 11 by means of hollow forwardly extending attachment members 20 which are attached to pivotal mountings 21 which are in turn pivotally secured to a cross member 60a which is rigidly secured to the frame members 10 and 11. The attachment members 20 and the rear wheel mounting hubs 12 and 13 may be formed separately or as an integral unit, and the hollow attachment members 20 operate to convey cooling air to the motors 16 to 19 attached to the hubs 12 and 13.
Each pivotal mounting 21 includes a top transverse journal 22 and a bottom longitudinal journal 23 which allows two rotational degrees of freedom for the hubs 12 and 13. The transverse journal 22 engages spindle sections on the ends of a shaft 24 (FIG. 5) which forms part of the rear cross member 60a for the frame members 10 and 11, which, as shown in FIGS. 1 and 2, are of increased depth in this region to allow the cross member 60a to penetrate the side plates of the frame members 10 and 11, to which the cross member 60a is rigidly secured in some suitable manner. Each lower journal 23 receives the end portion of one of the attachment members 20, within which journal 23 this end portion is free to rotate, the end portion of the attachment member 20 being restrained within the journal by a flange 23a formed on the attachment member 20 and a nut 23b engaging the end portion of the attachment member 20, as shown most clearly in FIG. 6 of the drawings.
The frame members 10 and 11 also provide journals for mounting members 26 for vertically compliant suspension means or spring units 27, the mounting members 26 also having integrally formed journals 28 which receive pivot pins 29 about which the body B of the truck also pivots. The spring units 27 include cylinder housings 41 rigidly attached to the mounting members 26 and receiving piston units 40 which are attached to the mounting hubs 12,13 by rearward extensions 42. The spring effect may be obtained by the compression of a gas within the cylinder or by other suitable means.
The mounting members 26 have downwardly extending rigid flanges 30 which are adapted to be engaged by a projection 31 extending from a respective hub 12,13 at a position closely adjacent the flanges 30 whereby transverse rotation of the hubs 12 and 13 is limited.
Rotation of the attachment member 20 and its connected hub 12,13 in the other direction is confined to the vertical longitudinal plane by its attachment to the piston 40 and the confinement of the piston 40 by the cylinder of the spring means 27 to travel along the axis of the cylinder. The attachment of the piston 40 to the rearward extension 42 is by a restrained spherical bearing 45 between the piston 40 and the rearward extension 42.
The above described coupling and spring unit for the rear wheel mounting hubs 12 and 13 allows the hubs 12 and 13 to pivot about both longitudinal and transverse axes to facilitate greater flexibility in the movement of each hub 12,13 with respect to the frame and truck body B. One such movement is illustrated in FIG. 4 of the drawings.